The present invention generally relates to semiconductor processing, and in particular to a system and method for characterizing chemical mechanical polishing (CMP) processes via wafer based temperature sensors.
As semiconductors have become more complicated (e.g., increasing number of interconnect layers), the planarization of dielectric and metal layers has become more important to achieving desired critical dimensions (CDs) in such semiconductors. One technique employed in the planarization of layers is chemical mechanical polishing (CMP). In general, CMP is a surface planarization technique in which a wafer is processed by a polishing pad in the presence of an abrasive slurry (although recent slurry-free techniques are also employed). One goal of CMP is more global planarization with stricter planarization tolerances and more repeatable results. In CMP, high elevation features are selectively removed resulting in a topology with improved planarity. Such removal is achieved, at least in part, via a combination of a chemical process and an abrasive process, both of which affect and/or are affected by the temperature of the wafer.
Some goals of CMP include achieving satisfactory planarity across a wafer, achieving desired film thickness uniformity, removing chemical reaction products and/or layers at a desired rate, achieving desired selectivity and/or endpoint detection and to not introduce defects into a wafer undergoing CMP. Whether these goals are achieved can depend on a variety of factors. Removal rate may depend, for example, on the type of material being removed, the relative velocity between the wafer and the abrasive pad, the temperature of the wafer, the slurry feed rate, the type of polishing motion employed, the slurry formula, the slurry pH, the concentration of solids in the slurry, slurry particle size, pad hardness and pad conditioning.
The mechanics of metal CMP include chemically forming an oxide of the metal on the metal film surface on the wafer. The oxide is then removed mechanically via, for example, abrasives in the slurry. The mechanics of other CMP (e.g., polysilicon polish, dielectric polish) similarly involve a chemical reaction followed by a mechanical removal of reaction products. The rate of the chemical reduction reaction, which facilitates selectively removing the metal films and/or other layers and/or reaction products during CMP, is strongly temperature dependant. Conventionally, such temperature, if measured at all, was measured indirectly via analysis of the temperature of the polishing pad(s).
The polishing pad facilitates precisely removing reaction products at the wafer interface to facilitate precise layer thickness production. For example, CMP processes can be employed to precisely remove around 0.5 to 1.0 xcexcm of material. The polishing pads may vary, for example, in hardness and density. For example, pads can be relatively stiff or relatively flexible. A less stiff pad will conform more easily to the topography of a wafer and thus while reducing planarity may facilitate faster removal of material in down areas. Conversely, a more stiff pad may produce better planarity but may result in slower removal in down areas. The degree to which the pad conforms to the topography can affect the friction between the pad, slurry and wafer, and thus can affect the temperature of the wafer. Furthermore, the polishing pads may glaze during processing of wafers, which again may affect the abrasiveness and thus heat generated by friction during CMP. For example, a new pad may achieve a removal rate of around 210 nm/min while a pad that has been employed to polish fifty wafers may only achieve a removal rate of around 75 nm/min. Thus, the rate at which CMP progresses may vary depending on the temperature of the wafer, which can be affected, for example, by the hardness, density and glazing of the pad employed.
The rate at which CMP progresses may also vary depending on parameters of the slurry employed. Slurries may consist, for example, of small abrasive particles suspended in a solution (e.g., aqueous solution). Acids or bases can be added to such solutions to facilitate, for example, the oxidation of the metal on the wafer and/or other chemical reactions involved in other non-metal CMP processes. Slurry parameters that may impact polishing rates include, but are not limited to, the chemical composition of the slurry, the concentration of solids in the slurry, the solid particles in the slurry and the temperature of the wafer to which the slurry is applied. Thus, once again, the temperature of the wafer is involved in the progress of the CMP.
Conventional CMP processes have either lacked control systems, requiring pre-calculated CMP parameters based on theoretical or indirect empirical data, or have had indirect control, which is based on indirect information (e.g., indirect temperature measurements of polishing pad). Such pre-determined, theoretical and/or indirect measurement based parameters do not provide adequate initialization and/or monitoring and thus do not facilitate precise characterization and/or control of the CMP process.
Fabricating an integrated circuit (IC) typically includes sequentially depositing conducting, semiconducting and/or insulating layers on a silicon wafer. One fabrication step includes depositing a metal layer over previous layers and planarizing the metal layer. For example, trenches or holes in an insulating layer may be filled with a conducting metal. After CMP planarization, portions of the conductive metal remaining between the raised pattern of an insulating layer may form, for example, vias, plugs and/or lines. The precision with which such vias, plugs and/or lines can be formed affects the achievable CDs for an IC, and thus improvements in characterizing and/or controlling a CMP process are desired.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description presented later.
The present invention provides a system and method that facilitates characterizing and/or controlling a chemical mechanical polishing (CMP) process by gathering wafer temperature information during CMP processing, where the wafer temperature is measured directly from sensors in the wafer. Thus, accuracy improvements over conventional systems that only indirectly measure wafer temperature by measuring the temperature of an abrasive pad may be achieved. Thus, the system includes wafer based sensors and apparatus to retrieve the wafer temperature from such wafer based sensors. One example of the system further includes a data store that can be employed to store data including, but not limited to, temperature information, slurry information, wafer information, motion (e.g., rotary, orbital, linear) information, pressure information and abrasive pad information associated with the CMP process being characterized. Another example of the system further includes a CMP control system that can be employed to analyze such temperature, slurry, wafer, pressure, motion, and/or pad information to facilitate characterizing a CMP process, to facilitate selecting CMP process parameters and/or for controlling, in-situ, a CMP process.
The present invention thus provides a technique to monitor the surface temperature of a wafer during CMP processing. The present invention can be employed in CMP processing of metal films including, but not limited to, copper (Cu), tantalum (Ta), tungsten (W), aluminum (Al) and titanium (Ti), for example. The metal film can be subjected to a chemical reaction (e.g., oxidation), where the chemical reaction is dependant on the temperature of the wafer and/or the metal film. The present invention can also be employed in CMP processing of layers including, but not limited to, polysilicon layers and dielectric layers. Since the polish rate is affected by the rate of chemical reaction, the polish rate is therefore affected by the temperature of the wafer and/or film. Thus, monitoring the temperature of the wafer and/or film can provide data that facilitates characterizing a CMP process and thus improving wafer quality.
In addition to measuring the temperature of the wafer, layer and/or metal film, the present invention facilitates measuring radial temperature gradients, which can facilitate improving within wafer planarization uniformity, with resulting improvements in wafer quality.
In one example of the present invention, an array of temperature sensors is integrated into a silicon wafer substrate to directly measure wafer temperature during CMP. To facilitate retrieving wafer temperatures, the substrate may include signal processing circuitry, a power source, an electrical temperature transducer and other components, for example.
In another example of the present invention, the system includes a wafer that has a metal layer and/or substrate and a temperature sensor located in and/or on the metal layer and/or a substrate. The system also includes a temperature monitoring system that can read the wafer temperature from the temperature sensors and that can analyze the wafer temperature to characterize the CMP process. Characterizing the CMP process includes producing information concerning factors including, but not limited to, polishing rate, polishing uniformity and introduction of defects during polishing. The factors can be correlated, for example, with polishing parameters including, but not limited to, polishing time, polishing temperature, polishing pressure, polishing speed, slurry properties and wafer/metal layer properties as related to wafer temperature information. For example, rotation speed, pressure and removal rate may be identifiable by the temperature of the wafer. Such characterization can be employed, for example, to facilitate initializing subsequent chemical mechanical polishing processes and/or apparatus and/or to control such chemical mechanical polishing processes and/or apparatus.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.